Electrochemical ammonia synthesis by reduction of nitrate on Au doped Cu nanowires

Electrochemical nitrate reduction reaction (NO3−RR) to synthesize valuable ammonia (NH3) is considered as a green and appealing alternative to enable an artificial nitrogen cycle. However, as there are other NO3−RR pathways present, selectively guiding the reaction pathway towards NH3 is currently challenged by the lack of efficient catalyst. Here, we demonstrate a novel electrocatalyst for NO3−RR consisting of Au doped Cu nanowires on a copper foam (CF) electrode (Au–Cu NWs/CF), which delivers a remarkable NH3 yield rate of 5336.0 ± 159.2 μg h−1 cm−2 and an exceptional faradaic efficiency (FE) of 84.1 ± 1.0% at −1.05 V (vs. RHE). The 15N isotopic labelling experiments confirm that the yielded NH3 is indeed from the Au–Cu NWs/CF catalyzed NO3−RR process. The XPS analysis and in situ infrared spectroscopy (IR) spectroscopy characterization results indicated that the electron transfer between the Cu and Au interface and oxygen vacancy synergistically decreased the reduction reaction barrier and inhibited the generation of hydrogen in the competitive reaction, resulting in a high conversion, selectivity and FE for NO3−RR. This work not only develops a powerful strategy for the rational design of robust and efficient catalysts by defect engineering, but also provides new insights for selective nitrate electroreduction to NH3.


Ammonia (NH
) is not only an essential chemical and the cornerstone of the large and ever-growing fertilizer industry, but also considered as an important energy storage medium and carbon-free energy carrier. [1][2][3][4] Currently, most of the ammonia synthesis in the world is implemented via the Haber-Bosch process, which consumes about 5.51 EJ of energy every year (∼38 GJ/t NH 3 ) and emits over 450 million metric tons of CO 2 (∼2.9 t CO 2 /t NH 3 ), this is because the process requires substantial driving force and hydrogen gas (e.g., H 2 ), which is produced from natural gas or coal through steam reforming, accounting for about half of CO 2 emissions in the entire process. [5][6][7][8] Nitrogen gas (N 2 ) from air was identied as one major nitrogen source for this renewable route via electrochemical nitrogen reduction reaction (NRR), however, the faradaic efficiency (FE) is greatly hampered by the high dissociation energy of N^N tripe bond (941 kJ mol −1 ) and poor solubility of N 2 in electrolytes and the competitive reaction of H 2 evolution. [9][10][11] While exciting progresses in NRR catalyst development have been made, in many cases it is still challenging to rmly attribute the detected NH 3 to NRR process rather than contaminations due to the extremely low NH 3 production rate (mostly <200 mg h −1 mg cat. −1 ). 12,13 Thus, developing a new route for ammonia synthesis under benign conditions is urgently desired.
It is common knowledge that, nitrate pollution in surface water and groundwater is widespread in the world. 14 High concentrations of nitrate in aquatic ecosystems pose a serious threat to ecological balances and human health. To minimize such adverse effects, many approaches including biological denitrication, 15 reverse osmosis, 16 ion exchange, 17 electrodialysis, 18 membrane ltration, 19 electrocatalytic denitrication [20][21][22] and so on have been adopted to dispose of nitrate contamination to produce clean water, among them, electrocatalytic denitrication driven by "green" electricity generated from renewable resources is the most likely practical alternative, which can overcome these limitations. Compared with the NRR, the nitrate reduction reaction (NO 3 − RR) to NH 3 is not limited by the low solubility of N 2 in water environment and its thermodynamically more favourable because of lower dissociation energy of N]O bond (204 kJ mol −1 ) than the N^N tripe bond (941 kJ mol −1 ). 23,24 Therefore, it is an frontier eld that needs indepth study.
Herein, we utilized a facile three-step method to fabricate the Au doped Cu nanowires on a copper foam (CF) (denoted as Au-Cu NWs/CF) electrode for the selective nitrate electroreduction to ammonia. The Au-Cu NWs/CF sample exhibited an exceptional performance with the NH 3 yield rate of 5336.0 ± 159.2 mg h −1 cm −2 and the FE of 84.1 ± 1.0% at −1.05 V (vs. RHE) for the electrocatalytic NO 3 neutral conditions. 15 N isotopic labelling experiments were performed to conrm the origin of ammonia, which was quantied by both 1 H nuclear magnetic resonance (NMR) spectra and colorimetric methods. The XPS analysis and in situ infrared spectroscopy (IR) spectroscopy characterization results indicated that the oxygen vacancies in Au-Cu NWs/CF can weaken the N-O bonding, 25 moreover, the electron transfer between Cu and Au interface could inhibit the competitive reaction of the hydrogen evolution reaction (HER), 11 resulting in high NH 3 yield rate and FE of NO 3 − RR. Fig. 1a shows the schematic illustration of the growth of the Au doped Cu nanowires on a copper foam electrode. As illustrated in Fig. 1a, Au-Cu NWs/CF can be prepared by a three-step method. In the rst step, the Cu(OH) 2 NWs/CF was prepared via a facile wet-chemical oxidation method. Subsequently, NWs/CF was directly immersed into 10 mM HAuCl 4 $3H 2 O solution for 12 h, dried at 60°C under vacuum for 4 h, the Au-Cu(OH) 2 NWs/ CF was annealed under Ar atmosphere to obtain Au-CuO NWs/ CF. Finally, the Au-Cu NWs/CF was obtained by in situ electrochemical reduction of the resultant Au-CuO NWs/CF. The scanning electron microscopy (SEM) images of CF ( Fig. 1b) and Au-Cu NWs/CF ( Fig. 1c) demonstrate that the nanowires have been successfully generated on CF. Aer cation exchange reaction with Au precursor and subsequent thermal treatment and electrochemical reduction, the morphology of nanowires was largely maintained on the Au-Cu NWs/CF with the diameters of ∼100 nm (Fig. 1c). Fig. 1d shows the X-ray diffraction (XRD) patterns of CF and Au-Cu NWs/CF samples. As shown, similar diffraction peaks at 2q = 43.3°, 50.4°and 74.1°can be observed for these two samples, corresponding to (111), (200) and (220) plane of metallic Cu (JCPDS no. 04-0836), respectively. [26][27][28] While besides of typical diffraction peaks of metallic Cu, the Au-Cu NWs/CF sample also displays the weak characteristic peaks of Au nanoparticles at 2q = 38.2°, 44.3°, 64.6°and 77.5°, suggesting the formation of fcc Au phase on Cu nanowires with low loading content. 29 The actual loading of Au was calculated to be 5.6 wt% by inductively couple plasma atomic emission spectroscopy (ICP-AES). High-resolution TEM (HR-TEM, Fig. 1e) images show the lattice fringes of 0.24 and 0.27 nm, corresponding to the (111) and (200) planes of Cu, respectively, in good accord with the XRD results. 27,28 In addition, the corresponding element mapping analysis of Au-Cu NWs/CF reveals that Au was homogeneously dispersed over the whole Cu foam (Fig. 1f).
The X-ray photoelectron spectroscopy (XPS) measurement was performed to investigate the surface composition and valence state of Au-Cu NWs/CF. For comparison, we also performed the XPS characterization of CF sample. The XPS survey spectra and high-resolution XPS spectra of Au 4f veried the existence of doped Au in the Au-Cu NWs/CF ( Fig. 2a and b). The high-resolution XPS spectra of Cu 2p in bare CF substrate is shown in Fig. 2c, where peaks of Cu 2p 3/2 and Cu 2p 1/2 appear at 932.5 and 952.3 eV. [26][27][28] The two characteristic peaks conrms the presence of Cu 0 /Cu 1+ . [26][27][28] Note that aer Au doping, the binding energy of Cu 2p 3/2 and Cu 2p 1/2 shied by 0.5 eV and 0.4 eV towards the lower binding energy of 932.0 and 951.9 eV in Au-Cu NWs/CF (Fig. 2c), due to the transfer of electrons  between Cu and Au via chemical binding, which led to an increase in charge density and is conducive to electrocatalysis. 27,28 Additionally, the new peak at binding energy of 934.2 eV was attributed to Cu 2+ in Au-Cu NWs/CF. Based on previous reports, 27,28 we further used Auger Cu LMM spectra to conrm the coexistence of Cu 0 and Cu 1+ . It can be clearly observed in the Fig. S1 † (ESI) that the Auger kinetic energy peak is wide and asymmetric in the range of 906 eV to 924 eV. The two asymmetric peaks with centers located at the position around 916.5 and 918.7 eV, 916.1 eV and 918.4 eV can be assigned to Cu 1+ and Cu 0 in the CF and Au-Cu NWs/CF, respectively. 27,28 In the O 1s XPS spectra (Fig. 2d), 530.9 and 532.5 eV, 530.6 eV and 531.8 eV correspond to lattice oxygen and oxygen vacancy in the CF and Au-Cu NWs/CF, respectively. 26 The signicantly increased oxygen vacancy aer doping is favourable for weakening the N-O bond and inhibiting the formation of byproducts in the electrocatalytic nitrate reduction reaction, thereby improving the selectivity of ammonia. 25 We . Chronoamperometry (CA) measurements of Au-Cu NWs/CF were conducted at different potentials for 2 h with continuous argon gas (Ar) bubbling. Fig. S5a † (ESI) shows the chronoamperometry curves at each given potential for 2 h electrolysis from −0.7 V to −1.1 V (vs. RHE). The concentration of NH 3 product was measured using indophenol blue method (Fig. S5b, ESI †). The calculated NH 3 yield rates and FEs based on three repeated experiments are given in Fig. 3b. It is worth noting that the Au-Cu NWs/CF achieved the highest NH 3 yield rate (R NH 3 ) of 5336.0 ± 159.2 mg h −1 cm −2 and the FE of 84.1 ± 1.0% at −1.05 V (vs. RHE). The selectivity of NH 3 (S NH 3 ) and R NH 3 show the same trend with the increase of potential, and highest S NH 3 was 90.6 ± 3.2% (Fig. 3c). In addition, the conversion of nitrate increases slowly with the increase of potential, and 100% conversion can be achieved at −0.95 V (vs. RHE) (Fig. 3d). When the potential further increased to −1.1 V (vs. RHE), the R NH 3 and S NH 3 decreased due to the competitive hydrogen evolution reaction (HER). 30 Although the electrodynamic potential of NO 3 − to NO 2 − is higher than that of NO 3 − to NH 3 , NO 2 − is easily detected an main by-product of NO 3 − RR. 31 As shown in Fig. S6   addition is also tested. The electrochemical measurement in blank 0.1 M Na 2 SO 4 electrolyte produced ignorable NH 3 (Fig. S9, ESI †), further conrming that the produced NH 3 orginated from nitrate electroreduction. The durability of the Au-Cu NWs/CF electrocatalyst for NO 3 − RR was subsequently assessed by consecutive recycling electrolysis at −1.05 V (vs. RHE), no noticeable decay in the cathodic current density and UV-vis absorptions (Fig. S10, ESI †). As shown in Fig. 3f, the R NH 3 and FE are stable aer 6 consecutive recycling tests, indicating the good durability of Au-Cu NWs/CF. Aer electrolysis, the high-resolution Au 4f and Cu LMM XPS spectra were carried out to analyze the electronic properties of Au-Cu NWs/CF before and aer NO 3 − RR measurement (Fig. S11, ESI †). Interestingly, aer NO 3 − RR, the Au 4f 7/2 shied slightly to a lower binding energy by 0.2 eV aer electrolysis (Fig. S11a, ESI †). Similarly, the Auger peak of Cu 2+ shied to a lower binding energy by 0.3 eV, while the Auger peaks of Cu 0 and Cu 1+ shied to the higher binding energy by 0.2 eV and 0.3 eV aer electrolysis (Fig. S11c, ESI †), indicating the existence of charge transfer between Au, Cu 2+ and Cu 0,+1 during NO 3 − RR process. Inaddition, the oxygen defect increased signicantly aer electrolysis (Fig. S11d, ESI †). In a word, the high electronic density of Cu 0 and oxygen vacancy decreased the reduction reaction barrier and inhibited the generation of hydrogen in the competitive reaction, resulting in a high conversion, selectivity and FE of Au-Cu NWs/CF for NO 3 − RR. 31,32 To gain a deeper understanding of the NO 3 − RR mechanism over Au-Cu NWs/CF catalysts, we utilized in situ infrared spectroscopy (IR) spectroscopy characterization to detect intermediates and monitor the reaction. Fig. 4a display the in situ IR spectra of Au-Cu NWs/CF under various potentials. As shown, without the applied potential, there is no any infrared peak in the in situ IR spectra. In the investigated potential range from −0.7 to −1.1 V (vs. RHE), the new infrared bands at ∼1541 cm −1 was assigned to the −NO x intermediates. 33,34 In addition, the bending mode of -NH 2 is also found at ∼1457 cm −1 . 34,35 Clearly, as the applied potential increased, the peak intensity of −NO x intermediates and -NH 2 gradually increased (Fig. 4a). Fig. 4b shows the in situ IR measurements for the NO 3 − RR at −1.05 V (vs. RHE). The IR intensity of the peaks at around 1457 cm −1 and 1541 cm −1 , corresponding to -NH 2 and −NO x intermediates is increased obviously from 4 to 36 min, implying that the NO 3 − RR takes place gradually with reaction time under the given electrocatalytic conditions. Evidenced by the in situ IR results, the NH 3 synthesis by NO 3 − RR is successfully achievable (Fig. S12, ESI †), supportable for the electrocatalytic experimental results aforementioned.
In conclusion, Au doped Cu nanowires on a copper foam electrode was synthesized via a facile three-step method, which further generated the oxygen vacancies in Au-Cu NWs/CF can weaken the N-O bonding, moreover, the electron transfer between Cu and Au interface could inhibit the competitive reaction, resulting in high conversion, selectivity and FE of Au-Cu NWs/CF for NO 3 − RR. The Au-Cu NWs/CF exhibited signicantly enhanced NO 3 − RR activity with an NH 3 yield rate of 5336.0 ± 159.2 mg h −1 cm −2 and the FE of 84.1 ± 1.0% at −1.05 V (vs. RHE) in neutral electrolyte. The in situ IR spectroscopy measurements conrm the successful realization of NH 3 synthesis by NO 3 − RR over Au-Cu NWs/CF. Our work would be helpful to design and develop high-efficiency NO 3 − RR electrocatalysts for ambient electrosynthesis of ammonia.

Conflicts of interest
There are no conicts to declare.